“Large Signal Polar Modulation” also known as “Envelope Elimination and Restoration (EER)” is generally known and applied for highly efficient linear power amplification of radio frequency (RF) signals having modulated amplitudes and phases. The basic principle of large signal polar modulation is well known in the art. The amplitude and phase modulated radio frequency signal is split into a constant amplitude radio frequency signal containing the phase information (PM) and a low frequency signal representing the amplitude information (AM). The constant envelope radio frequency signal is subsequently amplified in a high-efficiency non-linear amplifier. The signal is reconstructed again by collector modulation (or drain modulation dependent on the technology) of the final amplifier stage or stages by the low frequency signal representing the amplitude information. This is typically achieved by driving the final stages into compression such that the amplitude of the radio frequency signal passing through the amplifier stage is actually determined by the low frequency collector voltage (or drain voltage).
It is well known in the art that the linearity of a large signal polar power amplifier is limited by the imperfections during reconstruction of the output signal. During reconstruction, the amplitude modulated component AM and the phase modulated component PM have to be recombined in order to constitute an amplified version of the input signal. An ideal polar power amplifier would only change the amplitude of the amplified phase modulated radio frequency signal as function of the AM signal. However, in practical implementations, the AM signal affects also the phase of the amplified phase modulated radio frequency signal. This imperfection is quantified as the “AM2PM” transfer characteristic. The AM2PM defines the amount of undesired phase modulation due to the amplitude modulation during the signal reconstruction process. A second important characteristic is the amplitude-to-amplitude distortion which is usually denoted as “AM2AM”. AM2AM quantifies the relationship between the baseband envelope signal and the envelope of the radio frequency output signal. The deviation of the two envelope signals is due to distortion in the amplitude modulation process for example caused by a non-linear relationship between the collector (or drain) voltage and the envelope of the radio frequency output signal.
A major disadvantage of the conventional collector modulation (or drain modulation) approach consists in the limited linearity of the radio frequency power amplifier. The limited linearity impairs the dynamic range, i.e. the maximum range of the amplitude over which high linearity of the amplifier can be maintained. The upper limit of the linear range is typically limited by the supply voltage (e.g. battery supply voltage). The lower limit is defined by the minimum collector voltage (or drain voltage) of the amplifying transistors. Above the minimum collector (or drain) voltage, the undesired phase variation due to the AM signal applied to the amplifying transistor stages is still within the linearity limits. Below the minimum value the transistors show non-linear behavior, thereby introducing phase variation due to the AM signal causing distortion in the signal to be amplified. Another aspect that impairs the dynamic range is the reduced isolation of the radio frequency amplification stages. The lower limit of the amplitude of the output signal is determined by the attenuation of the constant envelope input signal by the power amplifier in the case that the AM control signal is set to its minimum value. The amplifier isolation is limited in this case by the parasitic capacitances of the switched-off transistors.
As the supply voltages decrease and modulation schemes become more complex for today's and future mobile low power applications, the requirements relating to linearity, noise and dynamic range of amplifiers become more and more relevant. For mobile equipment a typical battery supply voltage amounts to 3 V. A power amplifier using bipolar radio frequency devices provides a minimum useful collector voltage of not less than 0.3 V. This results in a maximum dynamic range of approximately 20 dB. However, the modern cellular systems require more than 40 dB of total dynamic range in order to cover the amplitude range of the modulating signal over the full range of average output power. The conventional approach to cope with these high dynamic range signals consists in a phase and/or amplitude pre-distortion in order to compensate the phase and/or amplitude distortion during the amplification and reconstruction process. However, this approach has a number of important disadvantages. The pre-distortion or pre-correction methods are either not applicable in certain operating conditions or at least not useful to compensate deviations in frequency, drive power, temperature, antenna impedance and supply voltage. Generally, the pre-distortion principles are not attractive for large dynamic range systems that cannot be compensated by (e.g. digital) pre-correction methods, as those methods are not sufficiently robust or not practical. Further, the conventional approach requires a complex design and additional calibration procedures during production if they are applied in combination with (e.g. digital) pre-distortion methods. This results in additional costs and in an increased time to market.